Aircraft use a variety of materials to provide energy to mobilize the aircraft. Often, aircraft use fuel tanks to supply a liquid fuel to an engine that allows mobilization of the aircraft. The fuel tanks typically have a maximum capacity that is fixed while a fuel held in the fuel tank may have a variable volume that changes during transportation and use of the aircraft.
Air-breathing-engine fuels (e.g., JP-10, JP-8, JP-7, and JP-5) used in various vehicles, such as aircraft (e.g., cruise missiles, UAVs, Drones, Decoys, and Aerial Target vehicles), have densities that vary as a function of temperature. This causes the fuel volume to expand as the system is exposed to heat or contract when exposed to cold temperature extremes. For example, over a change in temperature of 111.1° C. (200° F.) (e.g., −51° C. (−60° F.) to 60° C. (140° F.)) JP-10 fuel will change in volume approximately 8%. The volumes of JP-8, JP-7 and JP-5 vary by 10% or more over the same temperature range.
In the past fuel tanks have included ullage space within a fuel tank to allow for expansion and contraction of the fuel. This generally results in fuel volume constraints on the system due to tank sizing limits. In vehicle design a goal is to maximize fuel within the available volume. At a maximum temperature the fuel may expand to the maximum capacity of the fuel tank and the ullage would be at zero. As the fuel cools the volume of the fuel reduces and the volume of the ullage space increases.
Referring to FIG. 1, in the past a fuel tank 1 included fuel 2 and a rubber inner bladder 3 to interface between the fuel tank 1 and the exterior environment. The rubber inner bladder 3 prevents leakage of fuel from the fuel tank 1 and provides for ullage volume increase and decrease resulting from temperature variation. The rubber inner bladder 3 separates atmospheric air from the fuel 2 and acts as an expanding and contracting balloon that remains within the fuel tank 1 to provide for the ullage volume changes. The rubber inner bladder 3 cannot significantly protrude outside of the fuel tank 1 past a position shown under a high temperature condition 4. Rigid walls of the fuel tank 1 and a small opening to atmospheric air prevent the rubber inner bladder 3 from significantly protruding outside, which limits the total mass of the fuel 2 the aircraft can hold when subjected to the high temperature condition 4.
Under the high temperature condition 4 the fuel 2 is at a maximum volume and the rubber inner bladder 3 is at its smallest ullage volume. Under a medium temperature condition 5 the fuel 2 is slightly contracted and the rubber inner bladder 3 expands with air to fill the volume within the fuel tank 1 that was created by the contraction of the fuel 2 due to the reduction in temperature. Under a low temperature condition 6 the fuel 2 contracts more than the medium temperature condition 5 and the rubber inner bladder 3 expands with air further filling the available volume in the fuel tank 1. In all three temperature conditions 4, 5, and 6 the mass of the fuel is the same, but the volume the fuel occupies changes and the mass of air inside the fuel tank changes due to changes in the fuel's density. The fuel tank 1 with the rubber inner bladder 3 design must be under-filled at ambient temperatures to account for fuel expansion at higher temperature conditions, such as the high temperature condition 4, because the rubber inner bladder 3 cannot expand outward of the fuel tank 2. These higher temperature conditions generally occur during transport or storage, thus resulting in wasted ullage space within the fuel tank during operation in typical environments.
Normally refuelable fuel system include venting and refuel ports and are open to the environment. In some aircraft the fuel is isolated from the surrounding environment to isolate the fuel from water and air intrusion, which can cause the fuel to develop mold or allow fuel vapor to evaporate over time.